During World War II, Styer was deputy commander of the Construction Division in the Quartermaster Corps and later the Corps of Engineers. In March 1942, he became the chief of staff of the Services of Supply. He became deputy commanding general of the Army Service Forces in August 1943, as the Services of Supply was renamed. In this capacity he served on the Military Policy Committee, which oversaw the Manhattan Project.

In May 1945, Styer became the commanding general of Army Forces, Western Pacific. As such, he chaired the tribunal that tried and convicted Generals Tomoyuki Yamashita and Masaharu Homma for war crimes. He retired in 1947.

Styer graduated from Fort Humphreys in June 1920, and was posted to the Massachusetts National Guard in Cambridge, Massachusetts, where he was promoted to major again on 10 February 1921. He enrolled at the Massachusetts Institute of Technology in June 1921, and graduated in June 1922 with a Bachelor of Science in Civil Engineering. He returned to the Office of the Chief of Engineers in Washington, D.C., where he reverted to the rank of captain a second time on 4 November 1922. On 5 October 1925, he was posted to the New York Engineer District as executive officer and assistant district engineer. He was the district engineer from 16 June 1926 to 14 August 1926, and again from 30 June 1928 to 1 August 1928.[6]

From 11 December 1928 to 28 August 1931, Styer was an Engineer in Europe with the American Battle Monuments Commission. He returned to the United States, and was District Engineer of the Pittsburgh Engineer District from 16 September 1931 to 9 May 1936. He was then assigned as Assistant Engineer for Maintenance at the Panama Canal from 19 May 1936 to 17 July 1939. Nearly twenty years after he was first promoted to major, he was promoted to the rank of lieutenant colonel on 1 June 1937.[7]

From 4 September 1939 to 19 June 1940, Styer was a student at the Army Industrial College. He became the commanding officer of the 8th Engineer Squadron at Fort McIntosh, Texas, from 3 August to 24 November 1940, and was executive officer of the Engineer Replacement Center at Fort Humphreys and Fort Leonard Wood, Missouri. On 12 December 1940, he was assigned to the Construction Division in the Office of the Quartermaster General.[8] The US Army was about to embark on a national mobilization, and it was the task of the Construction Division to prepare the necessary accommodations and training facilities for the vast army that would be created. This enormous construction program had been dogged by bottlenecks, shortages, delays, spiralling costs, and poor living conditions at the construction sites, and newspapers published accounts charging it with incompetence, ineptitude, and inefficiency.[9]

A new head, Colonel Brehon B. Somervell, took over the Construction Division on 12 December 1940, and four days later Somervell reorganized it, bringing Styer in as his deputy, and replacing all but two of the Construction Division's branch heads, one of the exceptions being Colonel Leslie R. Groves, Jr.. One of Styer's tasks was finding talented officers to work on construction projects. His appeal to the Chief of Engineers released three officers, including Major Hugh J. Casey and Captains Edmund K. Daley and Garrison H. Davidson.[10] By December 1941, 375 projects had been completed and 320 were still under way, with a total value of $1.8 billion.[11] On 16 December 1941, the Construction Division was transferred from the Quartermaster Corps to the Corps of Engineers.[8] Styer, who had been promoted to colonel on 8 April 1941, drew up the plan for the transfer and the new organization,[12] in which he remained the deputy head. He was awarded the Army Distinguished Service Medal for his services. [8]

On 5 February 1942, the Chief of Staff of the United States Army, GeneralGeorge C. Marshall, announced that he was considering a radical reorganization of the War Department, and gave his staff 48 hours to comment on the proposals. Somervell, now the Assistant Chief of Staff, G-4, and Styer drew up a plan for a new, overarching logistics command initially known as the Services of Supply. Their proposal was accepted,[13] and the new command was created on 28 February 1942. [14] It was renamed the Army Service Forces in March 1943, as the term "supply" was felt to be too narrow a description of the broad range of logistical activities carried out by the organization.[15] Somervell became its commander,[16] and Styer his chief of staff from 9 March 1942. Styer was promoted to brigadier general the next day, and major general on 8 August 1942. He became deputy commanding general of the Army Service Forces on 13 August 1943.[8]

In September 1942, the director of the Office of Scientific Research and Development (OSRD), Vannevar Bush, suggested Styer be appointed director of the Manhattan Project, but Somervell did not wish to lose his services, and engineered the appointment of Groves instead. On 23 September 1942, Styer became a member of the Military Policy Committee, a group chaired by Bush (with James B. Conant as his alternate) with Rear Admiral William R. Purnell as its third member. As director, Groves was answerable to the Military Policy Committee, which was responsible for the higher direction of the Manhattan Project.[17] When the Combined Policy Committee was formed in September 1943 by the Quebec Agreement, its chairman, Secretary of WarHenry Stimson, had Styer appointed as his deputy. Styer also became chairman of its technical subcommittee, which included America's Richard Tolman, Britain's James Chadwick and Canada's C. J. Mackenzie.[18] The technical subcommittee became very influential, as it furnished most of the data on which the Combined Policy Committee based its decisions.[19]

In April 1945, Somervell sent him to the Philippines to report on preparations for Operation Downfall, the invasion of Japan. The Supreme Allied Commander in the South West Pacific, General of the ArmyDouglas MacArthur, asked Styer to become head of a logistics organisation for Operation Downfall. Styer accepted the offer, but when he returned in May 1945 to assume the post, he found it was not what he had accepted. Instead, he became the commanding general of Army Forces, Western Pacific, a command co-equal with Lieutenant General Robert C. Richardson, Jr.'s Army Forces, Middle Pacific, which included logistical units, but also combat forces, while the planning responsibilities remained with MacArthur's General Headquarters.[21]

On 24 September 1945, with the war over, MacArthur ordered Styer "to appoint military commissions for the trial of such persons accused of war crimes".[22] To try General Tomoyuki Yamashita, Styer appointed a commission of five general officers: Major Generals Russel B. Reynolds, Leo Donovan and James A. Lester, and Brigadier Generals Morris C. Harwerk and Egbert F. Bullens.[23] Styer had the authority to reduce the sentence handed down by the commission,[24] but he upheld the commission's death sentence on 12 December.[25] The sentence was upheld by the Supreme Court of the United States.[26] On 22 February 1946, Styer signed the order for Yamashita's execution, which took place the following morning.[27] General Masaharu Homma, the Japanese conqueror of Bataan and Corregidor, was subsequently also tried and convicted of war crimes committed by his troops during the notorious Bataan Death March, and Styer signed Homma's execution order too.[26]